This invention relates generally to the design of valves and, more particularly, to the design of a combination check valve, neutral valve, pressure rise rate valve and pressure relief valve for use in a hydraulic component such as a hydrostatic transmission or integrated hydrostatic transaxle using a closed hydraulic circuit to connect a pump and a motor.
Check valves, neutral valves, pressure rise rate valves, and pressure relief valves are known in the art. Generally, a check valve restricts fluid flow in one direction while a relief valve is used to reduce pressure spikes in the hydraulic circuit. A combination check valve and pressure relief valve is disclosed in commonly owned U.S. Pat. No. 6,691,512, the terms of which are incorporated herein by reference. A neutral valve will permit the flow of a hydraulic fluid from the closed circuit to a sump or reservoir when the component is at or near neutral to permit a wider neutral band. The use of neutral valves in a hydrostatic apparatus is disclosed in U.S. Pat. No. 4,674,287. Lastly, a pressure rise rate valve regulates how quickly pressure rises in a hydraulic system. In order to accomplish these functions in a single hydraulic component, it is generally required to use multiple valves.